Petrogenesis and Geodynamic Evolution in the Northern Westfjords, Iceland, Elucidated by Iceland's Oldest Silicic Rocks

Iceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present new zircon U-Pb ages bolstered with zircon trace element and isotopic compositions, and whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers-Hrafnsfjorour, Arnes, and Kaldalon-to understand the petrogenesis of large silicic volcanic centers in the northern Westfjords, Iceland. Our data confirm Hrafnsfjorour as the oldest known central volcano in Iceland (similar to 14 Ma) and establish an older age for Arnes (similar to 13 Ma) than previously estimated. We also report the first U-Pb zircon dates from Kaldalon (similar to 13.5 Ma). Zircon oxygen isotope compositions range from delta O-18 similar to+2 to +4 parts per thousand and indicate involvement of a low-O-18 component in their source magmas. Hrafnsfjorour zircon Hf (mean sample epsilon(Hf) similar to +15.3-16.0) and whole rock Hf and Nd (epsilon(Hf) = +14.5 to +15; epsilon(Nd) = +7.9 to +8.1) isotopic compositions are more radiogenic than those from Arnes (zircon sample epsilon(Hf) similar to +11.8-13; whole rock epsilon(Hf) = +12.8 to +15.1; epsilon(Nd) = +7.3 to +7.7), but Hrafnsfjorour whole rock Pb isotope compositions (Pb-208/204 = 37.95-37.96; Pb-206/204 = 18.33-18.35) are less radiogenic than those from Arnes (Pb-208/204 = 38.34-38.48; Pb-206/204 = 18.64-18.78). Kaldalon has zircon Hf isotope compositions of epsilon(Hf)similar to+14.8 and 15.5 (sample means). These age and isotopic differences suggest that interaction of rift and plume, and thus the geodynamic evolution of the Westfjords, is complex. Isotopic compositions of Hrafnsfjorour and Arnes support involvement of an enriched mantle (EM)-like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle. Plain Language Summary Iceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present data from the trace mineral zircon, as well as whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers-Hrafnsfjorour, Arnes, and Kaldalon-to understand the mechanisms of magma formation in the northern Westfjords, Iceland. Our data confirm Hrafnsfjorour as the oldest known central volcano in Iceland (similar to 14 Ma) and establish an older age for Arnes (similar to 13 Ma) than previously estimated. We also report the first dates from Kaldalon (similar to 13.5 Ma). Zircon oxygen isotope compositions suggest that silicic magmas in this area form from incorporation of at least a minor amount of pre-existing crust material. Isotopic compositions from Hrafnsfjorour and Arnes differ significantly, despite those volcanoes being only similar to 30 apart. The age and isotopic differences suggest that interaction of the Icelandic rift system and the Iceland mantle plume, and thus the geodynamic evolution of the Westfjords, is complex and doesn't follow a simple rifting scenario. Isotopic compositions of Hrafnsfjorour and Arnes rocks support involvement of an enriched mantle (EM)-like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle.